PROJECT SUMMARY There are many factors that govern the potential for arbovirus emergence and expansion, including host factors and regional environmental conditions, yet the foremost drivers of emergence and spread are the interactions between virus and vector. West Nile virus (WNV; Flaviviridae; Flavivirus) remains the most geographically widespread arbovirus globally and the most widespread and prevalent arbovirus in the U.S, including in New York State (NYS). A resurgence in WNV activity was seen in NYS in 2010 and nationwide in 2012, and our retrospective genetic studies now demonstrate that this was concurrent with genetic change. Prevalence of WNV in NYS mosquitoes last year was 2nd only to 2012, and WNV cases in upstate NY reached unprecedent levels. Although environmental shifts likely play a role in epidemiological patterns, our recent analyses have led us to hypothesize that genetic variation of WNV correlates to shifts in population- specific vectorial capacity, and that this contributes to WNV transmission and disease intensity. Our proposed studies aim to understand the phenotypic impact of WNV genotype and its role in prevalence and disease. We will combine widespread surveillance and deep-sequencing with comprehensive laboratory studies utilizing historic and contemporary isolates, reverse genetics and experimental studies in relevant genetically-distinct mosquito populations. These data together will provide a direct assessment of the role of emergent genotypes in driving WNV transmission and the importance of population-specific selection and competence. In addition, the comprehensive catalog of virus strains will allow us to launch detailed downstream mechanistic studies.